"What distinguishes a Grandmaster from a master?
Chess-lovers often ask questions like that. To many people it
seems that Grandmasters simply calculate variations a little
deeper. Or that they know their opening theory slightly better.
But in fact the real difference is something else. You can pick
out two essential qualities in which those with higher titles are
superior to others: the ability to sense the critical moment in a
game, and a finer understanding of various positional
problems." -- Yusupov, in Opening Preparation

" It is often supposed that, apart from their
'extraordinary powers of memory', expert players have phenomenal
powers of calculation. The beginner believes that experts can
calculate dozens of moves ahead and he will lose to them only
because he cannot calculate ahead so far. Yet this is utter
nonsense. From my own experience I can say that grandmasters do
not do an inordinate amount of calculating. Tests (notably de
Groot's experiments) supports me in this claim. If anything,
grandmasters often consider fewer alternatives; they tend not to
look at as many possible moves as weaker players do. And so,
perversely, chess skill often seems to reflect the ability to
avoid calculations. It is, in truth, not clear that chess is a
game of calculation. Of course there are times when intense
calculation is called for, and often the master is better at
dealing with these situations than the amateur. No wonder, he has
had more practise than the amateur, but all the same his innate
calculating ability need not be any greater. Most of the time it
is something quite different that is required in chess, something
more akin to 'understanding' or 'insight'. " -- David
NORWOOD, Chess and Education

"A lot of the difference between an IM and GM is a
seriousness to the game. The GM is willing to go through all
this. He's willing to put up with anything. This shows his
dedication. One other thing is the GMs superiority in tactics.
For example Christiansen can find tactics in any position. If
you're a GM you should be able to overpower the IM tactically.
The GM will often blow out the IM in this area. " -- Nick de
FIRMIAN, in How To Get Better at Chess : Chess Masters on Their
Art by GM Larry Evans, IM Jeremy B Silman and Betty Roberts

"One of the main aims has been to highlight the
differences in appraoch between a Grandmaster and a weaker
player, and to try and narrow the gap. To some extent this comes
down to technical matters - more accurate analysis, superior
opening knowledge, better endgame technique and so forth; but in
other respects the difference goes deeper and many readers will
find that they need to rethink much of their basic attitude to
the game. One example of this would be the tremendous emphasis
which is placed on the dynamic use of the pieces, if necessary at
the expense of the pawn structure, or even of material. This is
no mere question of style; it is a characteristic of the games of
all the great players." -- Peter Griffiths, Introduction to
Secrets of Grandmaster Chess.